Metal detector using cross-correlation between components of received signals

ABSTRACT

An apparatus is described for detecting metal objects in an interrogated environment. An alternating magnetic signal is transmitted, and received by a receiving coil. The signal is amplified, and is then synchronously demodulated based on a transmitted frequency. The synchronously demodulated signals are low-pass filtered, and then changes in the amplitude of these low-pass filtered signals are detected. The outputs are cross-correlated and interrograted, and are used to control a controlled phase shifter. The output of the controlled phase shifter is used as a reference input to a first one of the synchronous demodulators, and a shifted transmitted signal is used as a reference to the second synchronous demodulator.

This invention relates to metal detection apparatus and in particular tomaintaining substantial discriminatory sensitivity in mineralizedenvironments.

In this specification the term "metal detector" is used to refer todetectors of a type used for the purpose of discriminating metal withinthe ground but it can also be used to refer to a single detector or anarray of detectors where used to detect metal objects in soil on amoving conveyor system. Such a system may be used, for example, tolocate gold nuggets in soil moving on the conveyor system.

The invention is directed to some of the difficulties encountered whenusing a detector in the presence of ground containing varyingproportions of minerals magnetic characteristics of which may vary fromone location to another.

These minerals, such as ferrous oxides may produce strong signals in thedetector which make identification of a target signal (for example, asignal from a gold nugget) difficult to identify, that is these strongvariable ground signals can "mask out" the signal from a target object.The extent of this difficulty is very dependent on ground conditions.

There are a number of types of metal detectors each having a differentmethod of operation. There is however a common principle of operationcommon to all these types of detectors. This involves the production ofa magnetic field by a coil.

A conducting material within an effective range will interact with thisfield and change characteristics of the field which characteristics canthen be analysed.

This invention is directed to a type of metal detector known as analternating magnetic field metal detector where the field variations aresubstantially sinusoidal.

This will include detection electronic circuitry which compares thephase and magnitude of an emf signal induced in a receiver coil withthat of a transmitted signal.

The object of this invention is to provide means by which there can bemore effective detecting in a discriminatory way in such conditionsground containing mineralisation, particularly where the magneticcharacteristics are variable spatially.

It is well known that an induced signal in any material may have a phaserelationship that is useful in identifying it's magneticcharacteristics; this phase relationship can be referred to in terms ofquadrature components of the signal. These can be characterised by apurely reactive component and at 90 degrees to this, a purely resistivecomponent which is some times refered to as the loss component.

In general, mineralised ground will provide a large reactive componentand a small loss component whereas conducting objects, because of eddycurrents generated within them have large loss components by comparison.

It is this difference which allows for the possibility of locatingconducting objects even when mineralisation in the ground produces alarge return signal.

The best available conducting metal target detectors use two synchronousdemodulators known as the "X" and "R" channel, such that the "X" channelhas it's sensitive component axis aligned within a small angle of thepurely reactive retransmitted components, and the "R" channel has it'ssensitive component axis aligned within a small angle of the purelyresistive retransmitted components. The sensitive axis of the "R"channel can be manually varied in order to align it so that it is atquadrature with the ground vector. The detector is said to be goundbalanced for the areas of ground whose vector is aligned at quadraturewith the sensitive axis of the "R" channel. For these detectors the "R"channel is known as the "object channel". Ground balancing can also beachieved by adding a proportion of the "X" channel to the "R" channel bycontrol of the proportion added. For detectors using this latter groundbalancing method the composite signal is called the "object channel".Many detectors do not provide for either of these controls.

U.S. Pat. No. 4,128,803 uses an "X" and "R" signals channel and a thirddemodulator between these two channels to detect and identify thetarget.

U.S. Pat. No. 4,024,468 describes an attempt being made to discriminatebetween different types of metal objects by amplitude discrimination.

U.S. Pat. 4,099,116 and 4,300,097 uses a phase angle setting todiscriminate between various types of ferrous and non ferrous objects.Various feedback circuits from the "X" channel are used to compensatefor the mineralised component signal in the discriminator circuit.

U.S. Pat. No. 4,303,879 discloses an automatic tuning circuit betweenmode selection, with manual ground cancellation.

U.S. Pat. No. 4,344,034 uses a double detector circuit in which thesecond detector circuit attempts to eliminate ground signal from thefirst discriminating circuit.

In all these patents together with U.S. Pat. No. 4,507,612, U.S. Pat.No. 4,325,027, U.S. Pat. No. 4,470,015 and U.S. Pat. No. 4,514,692 theemphasis is in target identification in mineralised soil conditions.However the common difficulty encountered by all these techniquesconcerns the variability of the ground. The setting of the electroniccontrols which will, to a large extent, eliminate the interference fromthe ground have to be changed as the detector is moved from one regionto another and the ground mineralisation composition changes. Sometimesthis ground variation is minimal, slowly varying, and only substantiallyinvolves variations in the concentration of mineralisation, whereasother ground is also characterised by rapid spacial changes in the angleof the ground vector relative to the "pure " axis described above.

One well known technique to try and overcome this difficulty is to runthe detector in the "Auto" or "A.C." mode.

In this mode of operation low frequency components of the signal arefiltered out and in this way slow variations due to groundmineralisation are removed and only the high frequency components of thesignal (which may be due to the required object) remain.

Thus the compensating feedback circuits described in some of the abovepatents and other techniques employing high pass filtering (U.S. Pat.No. 4,470,015 and U.S. Pat. No. 4,507,612) and differentiating circuits(U.S. Pat. No. 4,514,692) are employed. U.S. Pat. No. 4,507,612 employshigh Q high pass filters to remove ground signals and then uses afeedback circuit to determine the phase angle of the target, for targetidentification purposes.

In all mineralised ground it is necessary to periodically manuallyadjust the ground balance control to minimise the signal due to thechanges in the mineralisation concentration for optimum performance.This procedure, although inconvenient, is satisfactory provided thecharacter of the mineralisation is constant over regions extending overmany meters.

In areas where the phase angle of the ground spatially varies rapidly,manual ground balance adjustment is impractical but, if not carried out,will result in large signals in the object channel due to changingmineralisation concentration, thereby substantially reducing the abilityto detect signals from target objects. These high frequency componentsof the ground signal can be much greater than remote or small targetsignals, and this is particularly the case near ironstone reefs wherethe adjustment may have to be made at intervals as close as one metre oreven less. Detectors with no means for ground balancing are particularlyinadequate in these conditions.

Prospecting under these conditions is extremely difficult and tediousand, despite their potential for producing gold, these regions areaccordingly avoided by prospectors.

An object to which this invention is directed then is to assist inreducing the above difficulties.

In accordance with this invention there is apparatus for detectingremote metal objects in soil by interrogation using substantiallysinusoidal magnetic transmitted signals for such interrogation theapparatus including a correlator, the apparatus being characterised inthat there are means to effect at least two signals derived fromincoming interrogation signal or signals by synchronous demodulationreferenced to the said transmitted signal, the apparatus being adaptedsuch that one of the said signals contains information distinctive ofchanges during different soil interrogation over time in soil backgroundmagnetic response and any target metal object at the transmittedfrequency, and the other signal will contain substantially thatinformation arising from changes over time in the said retransmittedmagnetic field from interrogation from a moving source of transmittedsignal relative to a target of any target conducting metal object, butrelatively little information arising from differences in the groundbackground magnetic response from interrogated location to location,means to provide for amplitude cross correlation between the two signalsby means of the correlator, the said correlator being adapted to alterthe said second signal in such a way that the said correlation betweenthe said signals is minimised within a selected response period as theapparatus interrogates different areas of soil locations.

In accordance with one form of this invention an alternating current isproduced in a transmitting coil producing a magnetic field whichinteracts with the immediate environment. The induced emf signal in areceive coil is demodulated with reference to the phase of thealternating current in the transmitting coil to produce an "R" channeland an "X" channel described above. The induced signal in the receivecoil results from two sources, namely, directly from the currentsflowing in the transmitting coil, and varying magnetic sources in thelocal environment under the influence of the transmitted magnetic field.

The transmitter coil and the receiver coil are normally set up so thatwhen the coils are remote from the ground or other objects the signalinduced in the receiver coil is substantially a minimum.

In this situation, it is said that the sensing coils are "nulled".

Consider for the sake of clarity, an ideal situation where thecapacitance between windings of both the transmitting coil and thereceiving coil is negligible. Also consider that the load presented tothe receiving coil by the detection electronics is effectively infinite.Furthermore consider that induced eddy currents in the transmitter orreceiver coil are negligible.

Thus for purposes of explanation only, in the description which follows,the receive signal may be considered as the induced emf resulting fromthe alternating flux due to the transmitting coil. For each Fouriercomponent transmitted, the corresponding voltage component in thereceive coil has a phase angle of 90° relative to the current in thetransmitter coil. This induced component will be called the purelyreactive component, where the measured, so called "X" component has acomponent vector within 10 degrees of the purely reactive component. Anyinduced received Fourier components with the same (or opposite) phase asthe transmitted current will be called purely resistive components,where the measured so called "R" component has a component vector within10 degrees of the purely resistive component.

Received signals resulting from local environmental sources in generalinduce both resistive and reactive components in the receiving coil. Twosources dominate in most ground.

One is from ferrous oxides which have a reactive component much largerthan the resistive component, and the second from mildly electricallyconductive sources such as moisture containing salts, clays and carbondeposits, all of which have small reactive components and largeresistive components at audio frequencies. In ground containing veryheavy ferrous oxide deposits (heavy ironstone deposits) the reactivecomponent can dominate the resistive component by as much as 100:1.

Usually the resistive component variations of the second source above inthe ground is not correlated with the reactive component variations.However, the resistive component of the ferrous oxides is correlatedwell with its reactive component.

The invention described and claimed herein comprises metal detectorapparatus incorporating a system for correlating changes in the "X" andobject channel signal with time, as the sensing coil passes from onearea of soil to another, on a continuous or repeating basis to produce acorrection signal which can be used to minimise those componentvariations in the "X" signal correlated with those simultaneousvariations in the object channel signal in the object channel. Changesover time in the "X" and "R" or object channel can be determined by theuse of high pass filters.

The method for effecting ground balancing can consist of either a systemfor automatically rotating the reference phase angle for the "R"component, or shifting the phase of the signal from the receive coil, ormultiplication of the "X" component by a factor determined by thecorrelated signal and subtracted from the "R"component signal.

By this means the detector is continuously adjusted to substantiallyreject signals in the "R" channel arising from changes in the magnitudeof the mineralised component in the ground as the character of themineralisation changes. The strength of the interrogated signal in handheld detectors is modulated by the swinging action of users as the headis swept across the ground from side to side, thereby enhancing thesechanges in the interrogated magnitude of the ground signal, but thisaction is by no means a necessary requirement for this continuous groundbalancing system using the method of correlation, as the groundinhomogeneities produce variations in the received signal's magnitude.

The advantages of using a time averaged correlated signal betweenchanges in the reactive and object channels are that the detectormeasures the amount of predominant ground signal present in the objectchannel and rapidly and continually adjusts this signal to zero.Furthermore the correlator technique will adjust out this groundcomponent with an accuracy which is comparable with the most carefulmanual adjustment and with a speed which is many times faster.

Difficulties can arise in providing a continuing ground balancing effectusing the principle of correlation, namely getting a feedback loop torespond quickly but maintain its stability for a wide range of groundconditions.

This problem can arise out of the fact that the strength of theintegrated correlation signal will be proportional to the strength ofthe reactive signal. A small error in the ground balance control in thepresence of a large reactive signal will produce a large component inthe object channel giving rise to a large correcting signal. This willlead to instability if the loop gain is too high; indeed, if the loopgain is too high, the system displays many typical characteristics ofnon-linear systems, such as bification. When ground containing lowconcentrations of ferrous oxides is encountered the reactive signal isproportionally small and large errors in the degree of ground balanceproduce only a small component in the object channel giving rise tosmall correcting signals. This produces a very long time constant in theloop if the loop gain is too low, and although the loop is stable, anunacceptable delay is produced in correcting the ground balance.

The problem can be overcome by providing means to detect the extent oftotal reactive signal and means to adjust loop gain so that with highermagnetic concentration in the target ground location the loop gain willbe accordingly reduced.

This can be achieved as follows: the signal from the reactive channel Xor high passed reactive signal Xa is rectified and passed to a peakdetector to determine the strength of the reactive signal. The peakdetector can have a droop time constant of the order of several seconds.A peak detector is more satisfactory than a low pass filter with asimilar time constant because, unlike the filter, transient responses in"X" do not cause momentary instabilities since the gain is immediatelyreduced. The output of the peak detector is fed to a divider as thedivisor which attenuates the loop gain. The operation of this divider issuch that the divisor has a lower limit, thereby not enabling divisionby zero.

With reference to FIG. 1 it is possible to set the "constant ofproportionality" of the divider such that the "gain" of the high passedobject signal, "Rm", fed through the divider can be considered constantfor values of the divisor above its preset lower limit. In this instancethe gain of "Rm" is defined as the pulse response magnitude of "Rm" to aspecific step response in the reactive component for a specificmisalignment of the "R" demodulator to the ground vector. Thus the gainof the feedback loop responsible for the "automatic ground balance" isindependent of ironstone concentration if the divisor is greater thanits preset lower limit, and the "free-air" value of "X" much less thanthe operating value of "X".

The invention may be better understood by reference to a preferredembodiment which shall now be described with reference to theaccompanying drawings wherein

FIG. 1 shows a block drawing illustrating the arrangement for a firstembodiment.

FIG. 2 is a block drawing showing the arrangement for a secondembodiment and

FIG. 3 is a graphical display illustrating some typical outputs.

FIG. 4, FIG. 5 and FIG. 6 are each circuit details of the preferredembodiment the circuits shown being in each case a circuit partinterrelated to the other parts by having common letters identifyingcircuit lines intended to be connected together.

The first embodiment as shown in functional blocks in FIG. 1 involvesthe use of a correlator for correcting the ground signal component inthe object channel, "Rm". The signal is first pre-amplified by amplifier21. "R" and "X" signals are produced by demodulators 1 and 2. Thecircuit is thus arranged that each signal passes through a pair of lowpass filters substantially matched in temporally response 4 and 5 with a(D.C.) output signal for sensing the presence of an object being takenfrom output 4,5.

From the low pass filters 4 and 5 the signals are passed through a pairof high pass filters 7 and 8 substantially matched in temporal response,giving Xa and Ra, with the reactive component Xa passed through either ahigh gain amplifier, or a comparator 14 to produce a signal of fixedamplitude but whose sign depends on the sign of Xa. The resistivecomponent Ra passes to a subtractor 11 where a multiple of the reactivecomponent Xa from multiplier 12 is subtracted from it.

The purpose for the subtractor will be explained later.

The resistive component is then passed as the dividend to a divider 13,where the divider is a processed signal from the reactive channel. Thequotient from the divider is then multiplied by the output ofcomparator/amplifier 14 in multiplier and then passed through a gaincontrol 16, to an integrator 17. The output of the integrator isconnected to voltage controlled phase shifter 18 to control the phaseset relative to the transmitted phase to the demodulators 1,2, and 3based on the input voltage to the voltage controlled phase shifter 18.The 90° phase shift required for the channel of X demodulator 1 and forthe input signal to the demodulator 3 comes from the fixed phase shifter19.

The sections of the circuit comprising 3, 6, 9, 10, 11, 12, 13, are forthe purpose of stability and time constant control in the feedback loop.

An additional problem arises out of signal components in "Ra" whichresult intrinsically from the effects of changing the phase angle of the"R" demodulator for a non-zero value of "X". Consider for example thesituation where the magnetic environment of the sensing coil is steadyand the value of "X" is non-zero. If the phase reference Rd referencedto the transmitted signal is shifted, the value of "R" changes owing tothe change in the projection of the unbalanced signal at the demodulatorinputs on the sensitive vector axis of the "R" demodulator. Thesecomponents detract from the intrinsic correlation process as they areunrelated intrinsically to the correlated ground components and can beeliminated by subtracting a signal "S" from "Ra" to give "Rm" which isfree of these components.

The method for removing these components is as follows. Through theaction of the demodulator 3, S is proportional to the sine of the phasedifference between the induced component Xd of the phase reference, andthe transmitted phase (here the "d" indicated the digital nature ofthese signals), such that S=0 if Xd and the transmitted current phaseare 90° out of phase, and S is a maximum for the in-phase condition. S'is the signal after passing through low 6 and 9 pass filters matched intemporal response to the filters in the "X" and "R" channels, so that S'represents the change in the phase angle.

Thus the output of the multiplier 12 is proportional to the product ofS' and X=S" as indicated, which in turn is proportional to the intrinsicsignal to be removed from "Ra". Subtraction of S" from "Ra" (scaledappropriately) therefore removes this unwanted component of Ra.

Note: "Ra" and S" can be correctly scaled by making sure Rm is a minimumfor changes in the phase of Rd, for a fixed input signal that results innon-zero value of X.

Note that the part of the system intended to remove signals in "Ra"intrinsic to variations of phase in Rd (viz; the "S" channel) may beomitted, but with a corresponding reduction in performance.

There is a yet further problem in providing continuously groundbalancing detectors using the principle of correlation, namely as thesensing coils pass over a target object in the ground, the presence ofthe "X" and "R" signal from the target modifies the measured "X" and "R"signal from that of the local soil. Indeed it is precisely thismodification which is used to detect the presence of an object. Toovercome the difficulty of the correlator system attempting to correctthe phase angle of the "R" channel owing to the measured resultant ofthe ground plus object, thereby unbalancing the detector from the groundbalanced phase position, the integrator is inhibited by a switch 46. Theswitch is opened if the averaged absolute value of Rm is exceeded by afixed multiple of the absolute value of Rm. This relative level sensingof Rm is processed in the object sensor 20.

The fixed 90° phase shifter in FIG. 1 may be some other angle than 90°,for example, 45°.

This may be advantageous in situations where the sought after objectshave a specific loss to inductive ratio, for example 45 degrees, inwhich case the correlator does not attempt to correct for the object,but rather, to a first order approximation ignores it and is onlysensitive to the ground.

It should be noted however that the elimination of the unwantedcomponents intrinsic to the process of changing the phase angle, is mostaccurately achieved when the phase is set approximately 90° relative tothe transmitted phase plus whatever phase shift is caused bypreamplifier 21.

The control system described above with reference to FIG. 2 whichadjusts the variation of the gain of the feedback loop in FIG. 1 can beperformed manually for "manual" detectors. In this instance, theoperator perceives the tuning-in time or instability of the detector andadjusts the feedback gain accordingly.

This does however require that the operator perform adjustmentsregularly in relatively inhomogeneous ground for optimum results.

In practice this procedure is not desirable because if the reactivecomponent suddenly increases, the feedback loop can become unstablewhich may result in a substantial rapid phase angle change. Uponreducing the feedback gain to regain stability, a lengthy correctionperiod is required.

Using the control described above, advantageous results can be madepossible, and the tuning-in time can be set by the operator to obtainthe best compromise between the tuning-in time and stability.

This adjustment can be achieved by varying the fine gain adjust 16 inFIG. 1.

The A.C. output of the detector is shown as output 2 in FIG. 1 where thephase correction "noise" has been removed from the "R" signal. Note: theRC decoupling combinations simply remove d.c. offsets from the divider,and depending on the technology, may not be necessary. In addition someof the functional blocks may be placed in different positions, forexample the multiplier 15 may be placed between the subtractor 11 andthe divider 13, without changing the principle of operation.

The system can be realised in many different types of electronictechnologies, for example FIG. 2 shows the correlation principleemployed to determine the amount of high passed "X" signal present inthe high passed "R" channel and then subtracting this component directlyfrom the high passed "R" channel without changing the phase angle.

This procedure requires very high precision with a large dynamic rangeto cover all possible ground conditions and can be implemented (thoughnot necessarily) with digital techniques using a microprocessor. Thesignal from the "X" and "R" channel are demodulated with demodulators 1and 2 and pass through low pass filters 4,5 to prevent aliasing and arethen converted to digital form using high precision analog to digitalconverters 3,6. The signals are then (digitally) filtered in filters 7,8and a fraction of the "X" channel signal determined by the correlation)is then (digitally) subtracted in subtracter 11 from the "R" channelwhich is then (digitally) low pass filtered 14 to reduce noise toproduce the output channel.

The correlator operates in the same way as previously described with thesign of the "X" channel being determined by sign sensor 14, and beingmultiplied by the processed "R" channel in multiplier 15, and integratedin integrator 17 to determine the extent of the "X" channel remaining inthe "R" channel. The usual sensing procedure using rectifier/peakdetector 10 is used to determine and correct using divider 13 for thegain in the feedback loop. Again the integrator is inhibited by the sameprocedure described above, that is when the output signal deviates fromzero by a statistically significant amount as detected by object sensor20, the integrator is inhibited by the opening of the switch 46.

The functional blocks can again be shifted without changing the basicprinciple of operation. For example, the division process can preceedthe correlation process before the integration process. Here themicroprocessor can be used to advantage by the implementation of various"adaptive" processes such as filters which continually adjust accordingto the various ground conditions.

FIG. 3 shows a typical time trace for the "X" and "R" channels for thedetector as the correlator corrects for the "X" component present in the"R" signal. Note the strong correlation between the resistive channeland the reactive channel at the start of the trace. As the phase controladjusts the phase angle, the component of "X" in "R" is reduced toalmost zero with only the "true" resistive component remaining.

Some dry sandy beaches and other soils may produce very littleinterfering signal, while other ground conditions containing perhapsheavy ironstone (ferrous oxide) concentrations, and/or mildlyelectrically conducting components such as some clay deposits, moistsalty conditions, produce severe interference, making detection ofvaluable objects in such soils difficult and in some cases almostimpossible.

One reason for the importance of obtaining a detection system which canoperate under these conditions is that such conditions are oftenencountered when prospecting for gold. They result from the weatheringof the ironstone-quartz reefs which contain the gold. The ironstonemineralisation is distributed in wide areas around the reef and is boundin the clays and soils which often contain the gold nuggets originatingfrom the reef. There are well known techniques for minimising thisinterference from the ground which will become apparent as the contentof this invention is explained.

The input signal from the receiver coil Rx goes to the circuit 21consisting of two amplifiers 22 and 23 shown in FIG. 4. The demodulators1 and 2 switch the signal using MOSFET switches 24 and 25. Switch 24produces the reactive channel and switch 25 produces the resistivechannel.

After demodulation the signal passes through low pass filters 4, 5 whichremove high frequency components of the switching.

From filter amplifiers 27 and 28 the signal then passes to high passfilters 7 and 8. Filter 7 consists of a single pole RC filter followedby a high gain amplifier 30 with a filter 8 having analogous structureincluding amplifier 31. The D.C. output signal is taken from the outputof amplifier 28 (output 1). From the high pass filter 8 (Line C) thesignal goes to subtracting circuit 11 shown in FIG. 5 which uses anamplifier 54 operating as a summing amplifier.

As the phase of the signal is reversed in the S circuit the summingamplifier 54 subtracts unwanted phase shift signal from the "R" channel.The signal from high pass filter 7 and amplifier 30 (FIG. 4) is furtheramplified at 53 and 52 (FIG. 5) to produce a clipped wave form tooperate switches 49 and 50. From the output of subtractor 11 an outputA.C. signal (output 2) can be taken from the amplifier 54. A signal isalso taken from the output of the "X" channel low pass filter 4 (line A)and goes to the peak detector and droop circuit 10. This signal is firstfull wave rectified by amplifier 33 and then the peak is determined bythe action of the amplifier 34 charging the capacitor 35 through adiode. The capacitor is discharged with a time constant t determined byresistive chain 36.

In this way the circuit "memorises" the magnitude of the reactive signal"X" which is present and this signal becomes the divisor of a dividingcircuit 13.

To obtain the required dynamic range, precision, and temperaturestability necessary for the division a digital/analog hybrid circuitdividing circuit 13 is used.

Two 4 bit up/down counters 41, 42 drive two 8 bit digital to analogconverters (D/A) 39,40 which use the same data bits. These two D/Aconverters track each other and one 40 uses a feedback circuit whichincludes the comparator 38. The sign of the comparator determineswhether the counters count up or down and in this way the output from 40tracks the output from 37.

The clock rate for the counters 41, 42 and the data latch 47 are takenfrom a divided frequency from the crystal oscillator circuit 71 (shownin FIG. 6).

The multiplying D/A 39 and associated operational amplifier in FIG. 5are connected as a dividing circuit with one input (dividend) comingfrom subtractor 54 and the other (divisor) from peak droop circuit 37through the 8 input bits. The output of the divider circuit passesthrough a high pass filter 43, 44 to a voltage follower driver 48, theoutput of which is switched by the switches 49,50.

The purpose of the high pass RC filter is to remove any D.C. componentfrom the divider.

The multiplying circuit 15 multiplies the signal from the divider by thesign of the high passed signal from the "X" channel through the actionof the inverting/noninverting switches 49,50 and amplifier 51.

This signal then passes to the integrator circuit 17 composed ofintegrating amplifier 45 together with an integrating gain controller16.

The integration process is inhibited if the FET switch 46 is opened.This is enabled by the action of the object detecting circuit 20, shownin FIG. 6, which has its input T, as the "AC" output. It comprises afull-wave rectifier 84 and 83, and an averaging circuit, namely, the lowpass filter 81. If a multiple of the absolute value of the input signalexceeds the averaged value, the comparing amplifier 82, which isconnected to integrator 17 produces a signal and opens the switch 46 andthus inhibits the integrator 17.

Line G from the integrator alters the phase angle of the demodulatingcircuits 1 and 2 by controlling the voltage controlled phase shiftingcircuit 19 of FIG. 4. Phase shifting circuit 19 is driven by the crystaloscillator 70 and dividing circuit 72 (FIG. 6) and consists of a phaselocked loop whose phase follows the phase of the current signal in thetransmitting Tx coil through the action of the resistance 59 andamplifier 62.

Driver amplifiers 55,56,57,58 in Fig. 4, drive the resonant tank circuitcomposed of the inductance of the transmitting coil and capacitor 60.

The value of capacitor 60 is adjusted to make the natural resonantfrequency of the tank circuit equal the driving frequency. Filter 73filters out the harmonics from the crystal oscillator frequency dividingcircuit 72 and drives the coil drivers (line R). Output from amplifier62 passes through the demodulator 63 which is integrated by integrator65 to control the switching level point on a quasi triangular wavegenerated from the oscillator 70 and by this means lock the phase of theoutput of 68 to the phase at the amplifier 62 (phase shift 90°) Latch 67is used to produce the clock.

The additional 90° phase shift from latch 69 drives the "R" demodulator.Phase shift is controlled by the voltage on line G which causes acurrent to flow to the virtual earth of the inverting input of theintegrator causing a corresponding shift in the D.C. component at thedemodulator 63 output to maintain locked phase. Thus the phase of thephase locked loop is shifted with respect to the output of amplifier 62.

The phase range can be centred with the potentiometer 66.

The demodulator 3 is also driven by the current sensing amplifier 62with the output of the demodulator passing through the low pass filter6and high pass filter 9. Line J from the high pass filter goes tomultiplier 12 which uses an A/D tracking converter which operates on thesame principle as the divider.

However in this case line J goes to the reference of the multiplying D/A76 where the signal is multiplied by the 10 bits to produce an outputsignal attenuated by 80 and going to line H. Tracking is accomplishedusing the comparator 74. Potentiometer 80 is used to change the gainfrom the multiplier and this is adjusted to null out the inherent phaseshift signal present in the "R" channel.

The claims defining the invention are as follows:
 1. Apparatus fordetecting remote metal objects in an interrogated environment by meansof magnetic interrogation, comprising:transmitting means fortransmitting an alternating magnetic signal; receiving coil means forreceiving induced signals, which are in part from the transmitting meansand from the interrogated environment; means, connected to the receivingcoil means, for amplifying the received signals. first and secondsynchronous demodulator means connected to said amplifier means, forsynchronously demodulating with respect to a transmitting frequency; afirst and second low-pass filter means, respectively receiving outputsof said first and second synchronous demodulator means; a first and asecond signal change determining means, respectively connected tooutputs of said first and second low-pass filter means, for detectingchanges in a magnitude of said first and second low-pass filter outputsignals; means for cross correlating outputs of said first and secondsignal change determining means; integrator means for integrating anoutput of said cross correlator means; controlled phase shifter meansreceiving an output of said integrator means, for producing a controlledphase shift based thereon, a signal provided by said transmitting meansbeing fed to an input of said controlled phase shifter means; wherein anoutput of said controlled phase shifter means is fed to a referenceinput of said first synchronous demodulator means, and first phaseshifting means coupled to the controlled phase shifter means forshifting the output thereof, and for providing the shifted signal as areference to said second synchronous demodulator means.
 2. A metaldetection apparatus according to claim 1 whereina phase shift of saidfirst phase shifting means is selected to provide said secondsynchronous demodulator a reference phase that is substantially in phaseor one hundred and eighty degrees out of phase with a received andamplified reactive interrogated component at an input to said secondsynchronous demodulator means.
 3. A metal detection apparatus accordingto claim 1 further comprising a sign detector means coupled between theoutput of said second signal change determining means and an input tosaid cross correlator means, where the output to said sign detectormeans is fed to the input of said cross correlator means, said detectormeans for providing an output signal that is constant in absolute value,and having a sign determined by the sign of an input thereof.
 4. A metaldetection apparatus according to claim 1, further comprising:rectifiermeans for rectifying an output of said second low pass filter means;peak determining means for determining a peak average of an output ofsaid rectifier means; divider means, receiving an output of said peakdetermining means as a divisor thereof and inserted between said crosscorrelator means and said controlled phase shifter means, an output ofsaid controlled phase shifter means being a dividend for said dividermeans and a quotient of said divider means connected to control saidcross correlator means.
 5. A metal detection apparatus according toclaim 1 further comprising:third synchronous demodulator means,receiving an output of said controlled phase shifter means as itsreference phase and receiving the transmitted signal at an inputthereof; third low-pass filter means, receiving an output of said thirdsynchronous demodulator means; third signal change determining means fordetermining changes in magnitude of an output signal of said thirdlow-pass filter means, connected to an output of said third low-passfilter means; multiplier means for multiplying an output of said thirdsignal change determining means by the output of said second low-passfilter means, said multiplier means receiving outputs of said thirdsignal change determining means and said second low-pass filter meansrespectively; subtractor means, for subtracting an output of saidmultiplier means from said output of said first signal changedetermining means, the output of said subtractor means being an inputfor said cross correlator means.
 6. A metal detection apparatusaccording to claim 5 wheresaid first, second and third signal changedetermining means are high pass filter means, each having substantiallythe same temporal characteristics, and said low-pass filter means eachhave substantially the same temporal characteristics.
 7. A metaldetection apparatus according to claim 4 where said peak averagedetermining means includes a peak detector, with a same signal sense asthe output of said rectifier means, and which has a decaying memory,which decays in time and is refreshed to a value of each peak thatexceeds in value a value of said decaying memory, wheresaid decayingmemory is constrained to exceed a fixed small value which has a samesign as a sign sense of the output of said rectifier means.
 8. A metaldetection apparatus according to claim 3 is further comprising:thirdsynchronous demodulator means, receiving the output of said controlledphase shifter means as its reference phase, and receiving thetransmitted signal as a signal to an input thereof; third low-passfilter means for filtering an output of said third synchronousdemodulator means; third signal change determining means which isconnected to an output of said third low-pass filter means, fordetermining changes in a magnitude of the output signal of said thirdlow-pass filter means; multiplier means, to which outputs of said thirdsignal change determining means and said second low-pass filter meansare fed respectively, for multiplying the output of said third signalchange determining means by the output of said second low-pass filtermeans; and subtractor means for subtracting the output of saidmultiplier means from said output of said first signal changedetermining means, the output of said subtractor means being an inputfor said cross correlator means.
 9. A metal detection apparatusaccording to claim 8 further comprising:a rectifier means for rectifyingan output of said second low pass filter means; peak average determiningmeans for detecting a peak average of an output of said rectifier means;divider means receiving an output of said peak determining means as adivisor thereof, and inserted between said subtractor means and saidcontrolled phase shifter means, an output of said subtractor means beinga dividend thereof and a quotient feeding said cross correlator means.10. A metal detection apparatus according to claim 8 wheresaid first,second and third signal change determining means are high-pass filtermeans each having substantially the same temporal characteristics; andsaid low-pass filter means each have substantially the same temporalcharacteristics.
 11. A metal detection apparatus according to claim 9whereinsaid peak average determining means includes a peak detector,with a same sign sense as an output of said rectifier means, which has adecaying memory, decaying in time and refreshed to a value of each peakthat exceeds in value a value of said decaying memory, where saiddecaying memory is constrained to exceed a fixed small value which hasthe same sign as the sign sense as the output of said rectifier means.12. An apparatus for detecting remote metal objects in an interrogatedenvironment by means of magnetic interrogation, comprising:transmittingmeans for transmitting an alternating magnetic signal; receiving coilmeans for receiving induced signals in part from the transmitting meansand from the interrogated environment; amplifier means, for amplifyingsaid induced signals; a first and a second synchronous demodulatormeans, for synchronously demodulating an output of said amplifier meanswith respect to a transmitting frequency; first and second signallow-pass filter means for respectively filtering the outputs of saidfirst and second synchronous demodulator means; first and second changedetermining means for respectively determining changes in a magnitude ofoutputs of said first and second low-pass filter means; controlled gainstage means, receiving an output of the second change determining means;subtractor means, receiving an output of the first change determiningmeans and an output of said controlled gain stage means; crosscorrelator means, receiving outputs of said second signal changedetermining means and said subtractor means; integrator means forintegrating an output of said cross correlator means, an output of saidintegrator means controlling said controlled gain stage means; phaseshift means producing a reference signal and providing it as a referenceto said second synchronous demodulator means, where said phase shiftingmeans being selected to provide said second synchronous demodulatormeans with a reference phase that is substantially in-phase or onehundred and eighty degrees out of phase with the received and amplifiedreactive interrogated component at an input to said second synchronousdemodulator means.
 13. A metal detection apparatus for detecting thepresence of conducting metal targets in soil according to claim 12further comprising: rectifier means for rectifying an output of saidsecond low pass filter means;peak average determining means fordetermining a peak average of an output of said rectifier means; dividermeans, receiving an output of said peak determining means as a divisorthereof, said divider means being inserted between said cross correlatormeans and said integrator means with an output of said cross correlatormeans being a dividend to said divider means and a quotient of saiddivider means feeding said integrator means, where said peak averagedetermining means includes a peak detector with the same sign sense asthe output of said rectifier means, and which has a decaying memory,decaying in time and refreshed to a value of each peak that exceeds invalue a value of said decaying memory, where said decaying memory isconstrained to exceed a fixed small value which has the same sign senseof the output of said rectifier means.
 14. A metal detection apparatusaccording to claim 12 where said first and second signal changedetermining means are high-pass filters which each have substantiallythe same temporal characteristics.
 15. A metal detection apparatus fordetecting the presence of conducting metal targets in soil according toclaim 12 further comprising:rectifier means, for rectifying an output ofsaid low-pass filter means; peak average determining means fordetermining a peak average of the output of said rectifier means dividermeans, receiving an output of said peak determining means as a divisorthereof, said divider means being inserted between said integrator meansand said controlled gain stage means, and an output of said integratormeans being a dividend thereof and a quotient controls said controlledgain stage means, where said peak average determining means includes apeak detector with a same sign sense as an output of said rectifiermeans, which has a decaying memory, decaying in time and refreshed to avalue of each peak that exceeds in value the value of said decayingmemory, where said decaying memory is constrained to exceed a fixedsmall value which has a same sign sense as the output of said rectifiermeans.
 16. A metal detection apparatus for detecting the presence ofconducting metal targets in soil according to claim 15 wheresaid firstand second signal change determining means are high-pass filter meanswhich each have substantially the same temporal characteristics, andsaid low-pass filter means each have substantially the same temporalcharacteristics.